1. Field of the Invention
This invention relates to vertical milling machines, and relates more specifically to the speed change control mechanism of a vertical milling machine which can be conveniently operated to stably shift the output power of the motor to the main shaft of machine through a low-speed transmission mechanism or a high-speed transmission mechanism.
2. Description of the Prior Art
The speed change control mechanism of a regular vertical milling machine, as shown in FIGS. 3 and 4, generally comprises a motor housing base 10, a motor 11 mounted in the motor housing base 10, a driving belt wheel 12 driven by the motor 11, a gear box 20 disposed at one side of the motor housing base 10. The gear box 20 comprises a driven belt wheel 14 fixedly mounted around a shaft 15 and turned by the driving belt wheel 12 through a transmission belt 13. The shaft 15 has a toothed portion 151 at one end, and is fixedly mounted with a gear 16 adjacent to the toothed portion 151. The gear 16 is meshed with a big Gear 17 for reduction of revolving speed. A pinion 21 is fixedly mounted around the gear shaft of the big gear 17 for engagement with a big gear 22, which is mounted on an auxiliary shaft 23, which is sleeved onto the main shaft 24 of the milling machine and locked in place. The auxiliary shaft 23 has a toothed portion 231 for engagement with the toothed portion 251 of the shaft 15. A bearing 251 is mounted on the auxiliary shaft 23 to hold a rack 2511. The rack 2511 is meshed with a toothed shaft 26. When the toothed shaft 26 is rotated, the rack 2511 is reciprocated, and the toothed portion 231 of the auxiliary shaft 23 is forced into engagement with the big gear 22 or forced away from the big gear 22, and therefore the revolving shaft is controlled. FIG. 3 shows the milling machine set at the high speed mode. As illustrated in FIG. 3, the big gear 22 is meshed with the pinion 21, and the gear 16 is meshed with the big gear 17. When the motor 11 is started, the output power of the motor 11 is transmitted through the driving belt wheel 12 and the driven belt wheel 14 to the shaft 15 and then to the main shaft 24 of the milling machine for a low-speed milling operation. When a high-speed milling operation is required, the toothed portion 231 of the auxiliary shaft 23 is forced into engagement with the toothed portion 251 of the shaft 15 (see FIG. 4) for direction transmission of the output power of the motor 11 to the auxiliary shaft 23 and the main shaft 24 without through the belt wheels 12 and 14. When at the high-speed mode as shown in FIG. 4, the auxiliary shaft 23 and the bearing 25 are forced upwards by the toothed shaft 26, and the toothed portion 231 of the auxiliary shaft 23 is forced into engagement with the toothed portion 151 of the shaft 15. The main drawback of this speed change control mechanism is that the main shaft 24 cannot be stably supported when at the high-speed mode. Because the size of the pinion 21 is about 21 mm, the gap is for example about 1 mm when the pinion is disengaged, and the depth of the engagement between the toothed portions 151 and 231 is about 3 mm, the up stroke of the auxiliary shaft 23 is as long as 18 mm. Therefore, the main shaft 24 tends to vibrate when it is operated at the high-speed mode.